A semiconductor device fabricating process for fabricating highly integrated semiconductor devices is required to use a gas not containing gaseous impurities and having a dew point of −50° C. or below. A dry dehumidifying device using a rotor provided with an adsorbent is used widely for producing clean, dry gas having such a low dew point.
A dehumidifying device for producing dry air having a low dew point is disclosed in, for example, JP 63-50047 B. The device is provided with a cylindrical honeycomb rotor formed by winding a single-sided corrugated board around an axis of rotation and having many parallel gas passages. The single-sided corrugated board is formed by processing a special paper sheet containing activated carbon and a ceramic material as principal components. Process gas passages, regenerating gas passages and virgin gas passages are formed separately around the axis of rotation. The process gas is passed through the rotating honeycomb rotor to dehumidify the process gas. Moisture removed from the process gas is carried away by the regenerating gas. This dry dehumidifying system has an adsorbent produced by fixing lithium chloride in pores of the activated carbon.
A dry dehumidifying system for producing dry air having a low dew point disclosed in JP 11-188224 A uses lithium chloride, silica gel or zeolite as an adsorbent.
A rotor of the dry dehumidifying device will be explained. The rotor is held rotatably in a case. A rotor and a case disclosed in JP 2001-276552 A will be described by way of example. Referring to FIGS. 11 and 12, a dry dehumidifying device 100 includes a case 110 provided with radial partition plates 111, and a rotor 101 rotatably held in the case 110. The rotor 101 is divided temporarily into parts by the partition plates 111 in an adsorbing zone S, a cooling zone T and a regenerating zone U as the rotor 101 rotates. Process air flows into the part of the rotor 101 in the adsorbing zone S through one of the opposite end surfaces of the rotor 101, for example, a front end surface on the front surface of the paper shown in FIG. 11.
The rotor 101 is formed by shaping a honeycomb base sheet 104 and has, for example, eight sectors 102 having a sectorial cross section and bearing an adsorbent. The sectors 102 are joined together in a cylindrical structure. The sectors 102 are reinforced by metal spokes 103. A metal rim 107 is put on the cylindrical structure formed by joining the sectors 102. As shown in an enlarged view in FIG. 13, each sector 102, the metal spoke 103 and the rim 107 are joined together or adhesively bonded together with a caulking material 120, such as heat-resistant silicone.
In the adsorbing zone S, moisture contained in the process air is adsorbed by the adsorbent borne by the rotor 101. Clean, dry air leaves the rotor 101 through the other end surface, namely, a front end surface on the back surface of the paper in FIG. 11, of the rotor 101. In the regenerating zone U, hot air heated at temperatures between about 180 and about 200° C. flows through the back end surface of the rotor 101 and flows through the rotor 101 to evaporate and carry away moisture adsorbed by the rotor 101 in the adsorbing zone S. The rotor 101 is exposed to hot air of a high temperature and is heated. If the hot rotor 101 moves through the adsorbing zone S, the process air cannot be dehumidified and the process air passed through the rotor 101 has a high dew point. Therefore, the cooling zone T is interposed between the regenerating zone U and the adsorbing zone S to cool the rotor 101.
Sealing members formed by laminating a PTFE film to a fluororubber sheet are attached to the partition plates 111 demarcating the adsorbing zone S, the regenerating zone U and the cooling zone T. The sealing members are pressed slidably against the end surfaces of the rotor 101 to prevent the leakage of the process air and the mixing of the process air and the hot air.
The dehumidifying device 100 has a limited dew point lowering ability. Usually, two dehumidifying devices are cascaded as mentioned in JP 63-50047 B. The two dehumidifying devices are connected such that the process air passed through an adsorbing zone in one of the two dehumidifying devices flows into an adsorbing zone in the other dehumidifying device. A dehumidifying system disclosed in JP 11-188224 A has three cascaded dehumidifying devices for further dew point lowering.
The semiconductor device fabricating process uses various chemicals, and produces a large variety of organic compounds including those of molecules having large diameters, those of molecules having small diameters, those having low boiling points and those having high boiling points.
Although the known dehumidifying devices are capable of satisfactorily adsorbing moisture, they are not satisfactory in organic compound adsorbing capacity. Thus, organic compounds contained in the process air could not be satisfactorily adsorbed by the known dehumidifying devices and some part of the organic compounds contained in the process gas is carried away by the process air leaving the dehumidifying device.
Accordingly, it is a first object of the present invention to remove both moisture and organic compounds.
Moreover, the semiconductor device fabricating process produces a large amount of gaseous, high-boiling organic compounds, such as DMSO (dimethyl sulfoxide), MEA (monoethanol amine) and HMDS (hexamethylenedisilazane). Although the known dry dehumidifying devices are able to adsorb those high-boiling organic compounds, the adsorbent of the dry dehumidifying device adsorbing those high-boiling organic compounds cannot be regenerated, because the adsorbent needs to be heated at a temperature on the order of 300° C. to remove the adsorbed high-boiling organic compounds from the adsorbent and the adsorbent is unable to withstand such a high temperature. For example, the highest temperature that can be withstood by activated carbon is on the order of 140° C. It is possible that activated carbon ignites if activated carbon is exposed to high-temperature air of temperatures exceeding 300° C. Therefore, activated carbon cannot be used. Silica gel cannot be used because the ability of silica gel deteriorates if silica gel is exposed to high-temperature air of temperatures exceeding 300° C. Hot air of very high temperatures exceeding 300° C. must be used to remove the high-boiling organic compounds in the regenerating zone U.
The caulking material 120, such as heat-resistant silicone, is unable to withstand high temperatures exceeding 200° C. The bonding ability or adhesive strength of the caulking material 120 caulking gaps between the metal spokes 103 and the sectors 102 deteriorates when high-temperature air of 300° C. or above flows through the rotor 101. Consequently, the sectors 102 fall off the rotor 101 and, in some cases, the rotor 101 becomes unserviceable. Since high-temperature air cannot be passed through the rotor 101, high-boiling organic compounds are continuously accumulated in the rotor 101 and the adsorbing ability of the adsorbent decreases accordingly.
Since the temperature of air that flows through the regenerating zone U of the rotor 101 is limited, the accumulated high-boiling organic compounds cannot satisfactorily be removed, the cleaning and dehumidifying abilities of the rotor 101 decrease and hence the rotor 101 needs to be replaced inevitably with a new one. A regenerating method of regenerating the deteriorated rotor 101 washes the rotor 101 with water. However, this regenerating method is unable to remove organic substances, particularly, high-boiling organic substances satisfactorily, and is unable to restore the rotor 101 to its initial ability. Water used for washing the rotor 101 needs to be treated by a waste water treatment, which is very expensive.
Accordingly, it is a second object of the present invention to provide a dehumidifying system using an adsorbent that can be regenerated by heat of a high temperature on the order of 300° C. after adsorbing high-boiling organic compounds.